The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
The present disclosure relates generally to transfer cases for use in four-wheel drive motor vehicles and, more particularly, to a transfer case providing an overdrive ratio.
Due to growing consumer demand for four-wheel drive vehicles, many different power transfer systems are currently utilized for directing power (i.e., drive torque) to all four wheels of the vehicle. For example, in some “part-time” power transfer systems, a transfer case is installed between the front and rear drivelines and is normally operable in a two-wheel drive mode for delivering drive torque to the driven wheels. When the four-wheel drive mode is desired, a mode shift mechanism is selectively actuated by the vehicle operator for directly coupling the non-driven wheels to the driven wheels for establishing a part-time or locked four-wheel drive mode. The mode shift mechanism may also be automatically controlled to provide drive torque to all four wheels “on-demand”.
A significant number of the transfer cases discussed above are equipped with a gear reduction unit and a range shift mechanism operable for permitting the vehicle operator to choose between a high-range and a low-range drive mode. The high-range typically directly interconnects the input shaft to the output shaft to establish a direct drive mode. The low-range drive mode is an underdrive ratio useful during low-speed off-road conditions.
In most four-wheel drive vehicles equipped with a transfer case, the transmission output shaft is coupled to the transfer case input shaft. However, in some applications, an add-on gearbox, commonly referred to as a compounder assembly, is installed between the transmission output shaft and the transfer case input shaft for providing an additional gear ratio, such as an “overdrive” ratio. An example of an add-on overdrive transmission assembly is shown in commonly-owned U.S. Pat. No. 4,798,103. Due to the long leadtime and cost associated with developing new multi-speed transmissions, the use of a compounder assembly in conjunction with an existing transmission for providing an additional gear ratio has found commercial success. One design compromise, however, is the additional length required in the vehicle's powertrain to accept installation of the compounder assembly.
From the foregoing, it is apparent that transfer cases equipped with two-speed gear reduction units have been successfully integrated into four-wheel drive vehicles. However, a need still exists to develop alternative gear driven units that further advance the technology in the field of four-wheel drive transfer cases.